Optical communication system

ABSTRACT

An optical transmission equipment in an optical communication system interconnecting two optical transmission equipment sets by a main transmission line and a backup transmission line. An optical amplifier amplifies and outputs optical signals from one transmission line in use, interconnecting the optical transmission equipment concerned with neighboring upstream optical transmission equipment, and outputs the optical signals including a signal component and a noise component. A controller controls an optical signal level so that a signal component in the optical signal reaches a predetermined level. The controller corrects the optical signal level when the transmission line transmitting the optical signal is switched either from the main transmission line to the backup transmission line or from the backup transmission line to the main transmission line, either between the optical transmission equipment concerned and a neighboring upstream optical transmission equipment, or between other two optical transmission equipment sets located on an upstream side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of applicationSer. No. 10/859,451 filed Jun. 3, 2004, now pending, and PCT ApplicationNo. PCT/JP01/10557 filed Dec. 3, 2001.

FIELD OF THE INVENTION

The present invention relates to an optical communication system andoptical transmission equipment therefor, and more particularly anoptical communication system having plural sets of optical transmissionequipment, of which the neighboring two sets are connected with the maintransmission line and the backup transmission line, and an opticalsignal is transmitted on either one of the transmission lines, andoptical transmission equipment therefor.

BACKGROUND OF THE INVENTION

In a single-wavelength optical transmission system transmitting datawith single wavelength light, there has been put into actual use asystem having duplicated (redundant) transmission lines, i.e. the maintransmission line and the backup transmission line, for a stationapparatus of a single (non-duplicated) configuration, thereby providingfault tolerance ability to the transmission line. Provision of faulttolerance ability with multiple configuration for each station apparatusmay be considered, but this is not adopted practically because costincreases.

FIG. 18 is a block diagram illustrating an overview of thesingle-wavelength optical transmission system having duplicatedtransmission lines. In this single-wavelength optical transmissionsystem, terminal stations 101, 103, and a relay station 102 are providedas station apparatuses. In some cases, more than two relay stations,instead of one, may be configured in the system.

The transmission lines provided between the neighboring stationapparatuses are duplicated. Between each station apparatus, maintransmission lines F01, F02, R01 and R02 and backup transmission linesF11, F12, R11 and R12, which are made of light fiber, are provided. Themain transmission lines are used normally, while the backup transmissionlines are used when a failure (for example, fiber break anddeterioration, etc. caused by secular change, etc.) occurs on the maintransmission line.

Each station apparatus includes optical amplifiers (AMP. For example,Erbium-doped fiber amplifiers: EDFA) 201-206, so as to amplify inputoptical signals. Further, each station apparatus includes opticalswitches (OSW) 301-308 for transmitting input optical signals to eitherthe main transmission line or the backup transmission line. The opticalswitches also receive the optical signals from either the maintransmission line or the backup transmission line. Since a sectionbetween the stations is termed as span, in some cases, optical switches301-308 may be termed as span switches.

These optical switches are switched to the main transmission line sidewhen there is no failure (normal state). In the event that a failureoccurs in the main transmission line, the corresponding optical switchesare switched over to the backup transmission line side. For example, inthe normal state, optical switch 301 is in the state switched to themain transmission line F01 side, and thereby an optical signal inputfrom optical amplifier 201 is output to the main transmission line F01side. In the event that a failure occurs in the main transmission lineF01, optical switch 301 is switched over to the backup transmission lineF11 side, and the optical signal from optical amplifier 201 is output tothe backup transmission line F11.

Meanwhile, as a communication system enabling high-speed transmission ofa large amount of data, a wavelength division multiplex (WDM) opticalcommunication system has received wide attention. This WDM opticalcommunication system multiplexes optical signals into differentwavelength signals, which transmits on one optical fiber.

In such a WDM optical communication system, implementation of a faulttolerant system is also desired. Similar to the single-wavelengthoptical communication system shown in FIG. 18, a system has been putinto practical use, such that optical fiber transmission lines betweenthe station apparatuses have duplicated (redundant) configurations,consisting of the main transmission line and the backup transmissionline, in contrast to the station apparatuses having single(non-duplicated) configurations, and that these transmission lines areswitched by the optical switches.

However, most of the conventional WDM optical communication systems havethe same number of optical switches as the number of multiplexedwavelengths, and each optical switch switches the optical signal on awavelength-by-wavelength basis. Accordingly, in the conventional WDMoptical communication system, with the increase of the number of themultiplexed wavelengths (for example, 160 waves, 320 waves, etc.), thecost has become increased because the provision of the optical switcheswith the same number as the number of the wavelengths is necessary.

Accordingly, there has been desired the WDM optical communication systemin which the WDM multiplexed optical signals are switched collectivelyby a single optical switch.

However, in order to put into actual use such a WDM opticalcommunication system using an optical, switch for collectively switchingoptical signals, there are some problems to be solved, differently fromthe optical communication system using the conventional optical switchswitching the optical signals on a wavelength-by-wavelength basis.

The first problem is how to compensate a transmission loss differencecaused by the transmission line (optical switch) switchover.

When the transmission line is switched over from the main transmissionline to the backup transmission line as a result of the switchover ofthe optical switch, an attenuation factor (loss) of the transmissionline is varied, producing a varied signal level to be input to anoptical amplifier in the succeeding stage. When the input signal levelis varied, the level of amplified spontaneous emission (ASE) light(noise component) generated from the optical amplifier also varies. Thisproduces an undesirable increase of possibility that the furthersucceeding optical amplifier becomes unable to amplify the signalcomponent (communication signal component) included in the input signalto a predetermined level.

In order to solve such a problem, conventionally, variable opticalattenuators have been provided on both, or either of, the maintransmission line and the backup transmission line. By adjusting theattenuation factor of any variable optical attenuator, the opticalsignal level input from the main transmission line to the opticalamplifier can be set substantially equal to the optical signal levelinput from the backup transmission line to the optical amplifier.

This method, however, has limit to cope with a variety of networkstructures. For example, this method is not applicable when the lengthsof the main transmission line and the backup transmission line largelydiffer, resulting in that the difference of the attenuation factorsbetween both transmission lines exceeds beyond an adjustable range ofthe variable optical attenuator. For this reason, it becomes necessaryto introduce another method, so that each optical amplifier can amplifyeach signal component to a predetermined level without adjusting theattenuation factor.

The second problem is how to prevent a receiving quality deteriorationof the optical signal caused by the transmission line (optical switch)switchover.

In some cases, an aspect of tilt produced in the optical signal isvaried when a wavelength dependent loss (WDL) of the transmission lineor the flatness in the gain of the intervened optical amplifier isvaried after the switchover of the transmission line from the maintransmission line to the backup transmission line, or vice versa, by useof the optical switches. This may produce an error in a particularwavelength signal at a receiving end, because of non-uniformity of thesignal quality being produced among the respective wavelengths.

To cope with this problem, it is necessary to prevent the signal qualityof each wavelength from deterioration even when the switchover isperformed, by compensating the difference in the tilt.

The third problem is how to prevent generation of a light surge.

The light surge is generated by a burst emission of the energyaccumulated in the optical amplifier (such as EDFA) during a lightinterception period of no optical signal input into the opticalamplifier, when the optical signal input is resumed after the lightinterception.

In the WDM optical communication system, the light interception isproduced when an optical signal is intercepted in the optical switchduring the switchover time (for example, ten and a few milliseconds),and accordingly no optical signal is input to the amplifier. Also, thelight surge is generated when the optical signal is input again to theoptical amplifier through the optical switch, after the switchovercompletion of the optical switch.

Especially in the WDM optical communication system, since the opticalsignals are multiplexed, output power (output electric power and outputlevel) from the optical amplifier becomes larger, in proportion as theincreased number of wavelengths. Because the light surge is superposedonto this output level, the resultant output level becomes remarkablyhigh, which may cause damage, such as melting of the optical fibertransmission line.

To prevent generation of such a light surge, one method may beconsidered so as to adjust the light interception period to a timesufficient for emitting the residual energy accumulated in the opticalamplifier. However, this method is not preferable because a longersignal interception period than a tolerable time is produced by theswitchover of the optical switch longer.

Accordingly, a method for preventing generation of the light surge,enabling the light interception period within the tolerable signalinterception period, is required.

SUMMARY OF THE INVENTION

It is an object of the present invention to compensate differences oftransmission line losses resulting from a transmission line switchover.

It is another object of the present invention to prevent deteriorationof receiving quality of an optical signal resulting from thetransmission line switchover.

It is still another object of the present invention to preventgeneration of a light surge caused by the transmission line switchover.

The first aspect of the present invention of the optical transmissionequipment in the optical communication system which has plural opticaltransmission equipments connected with their neighboring opticaltransmission equipments through main and backup transmission lines oneither of which optical signals transmit, is characterized by, havingoptical amplifiers in optical transmission equipments amplifying opticalsignal inputs from one transmission line in use for optical signaltransmission out of the main transmission line and the backuptransmission line interconnecting the optical transmission equipmentconcerned with neighboring upstream optical transmission equipment, andhaving controllers in optical transmission equipments controllingoptical signal levels of output from the optical amplifier so that thesignal component included in the optical signal output from the opticalamplifier reaches a predetermined level and correcting the opticalsignal level of output from the optical amplifier when the transmissionline transmitting the optical signal is switched either from the maintransmission line to the backup transmission line or from the backuptransmission line to the main transmission line, either between theoptical transmission equipment concerned and the neighboring upstreamoptical transmission equipment, or between other two opticaltransmission equipment sets located on the upstream side.

The first aspect of the present invention of the optical communicationsystem is characterized by, having one sending terminal station and oneor more relay stations and one receiving terminal station, transmittingoptical signals from the sending terminal station to the receivingterminal station through the relay stations, having the sending terminalstation which sends optical signals to one of the relay stations throughone of the main transmission line or the backup transmission line beingconnected to the neighboring downstream relay station, having relaystations which include an optical amplifier amplifying the opticalsignal input from one transmission line in use for optical signaltransmission out of the main transmission line and the backuptransmission line connected to the neighboring sending terminal stationor another neighboring upstream relay station outputting optical signalsincluding the signal component and the noise component and include acontroller controlling the optical signal level of output from theoptical amplifier so that the signal component included in the opticalsignal output from the optical amplifier reaches a predetermined level,and having the receiving terminal station which includes an opticalamplifier amplifying the optical signal input from one transmission linein use for optical signal transmission out of the main transmission lineand the backup transmission line connected to the neighboring sendingterminal station or the other relay station located on the upstream sideand outputting an optical signal including a signal component and anoise component; a controller in controlling the optical signal leveloutput from the optical amplifier so that the signal component includedin the optical signal output from the optical amplifier reaches apredetermined level. Each controller provided in a relay station and areceiving terminal station corrects the optical signal level of outputfrom each own optical amplifier when the transmission line transmittingthe optical signal is switched either from the main transmission line tothe backup transmission line or from the backup transmission line to themain transmission line, either between the optical transmissionequipment concerned and the neighboring upstream optical transmissionequipment, or between other two optical transmission equipment setslocated on the upstream side.

The first aspect of the present invention provides ability to compensatetransmission loss differences produced by the transmission lineswitchover by correcting the optical signal level.

In the preferred embodiment of the present invention, the controllerperforms the correction based on the first noise component included inthe optical signal being input to the optical amplifier and the secondnoise component generated by the optical amplifier concerned.

Further, in another preferred embodiment, the controller retains inadvance the correction data for performing the correction, and performsthe correction based on the retained correction data.

The second aspect of the present invention of optical transmissionequipment of an optical communication system having plural sets ofoptical transmission equipment of which each neighboring opticaltransmission equipment set is interconnected with the main transmissionline and the backup transmission line, is characterized by; an opticalamplifier amplifying optical signal inputs from one transmission line inuse for optical signal transmission out of the main transmission lineand the backup transmission line interconnecting the opticaltransmission equipment concerned with neighboring upstream opticaltransmission equipment, and outputting an optical signal including thesignal component and the noise component, a controller controllingoptical signal levels of outputs from the optical amplifier so that thesignal component included in the optical signal output from the opticalamplifier reaches a predetermined level, and a transmitter transmittingcorrection data for correcting output levels of optical amplifiers inthe neighboring upstream optical transmission equipment to theneighboring upstream optical transmission equipment.

The second aspect of the optical transmission equipment of the opticalcommunication system having plural sets of optical transmissionequipment of which each neighboring optical transmission equipment setis interconnected by the main transmission line and the backuptransmission line, is characterized by; an optical amplifier amplifyinginput optical signals and outputting optical signals after theamplification to the neighboring downstream optical transmissionequipment through one of the main transmission line and the backuptransmission line being connected to both the optical transmissionequipment concerned and the neighboring downstream optical transmissionequipment, and a controller controlling optical signal levels of outputfrom the optical amplifier so that the signal component included in theoptical signal output from the optical amplifier reaches a predeterminedlevel. When the transmission line outputting the optical signal isswitched either from the main transmission line to the backuptransmission line or from the backup transmission line to the maintransmission line, the controller corrects the optical signal leveloutput from the optical amplifier based on the correction datatransmitted from the neighboring downstream optical transmissionequipment.

The second aspect of the present invention also provides ability tocompensate transmission loss difference produced by the transmissionline switchover by correcting the optical signal level.

The third aspect of the present invention of the optical communicationsystem having a sending unit sending optical signals multiplexed withwavelength division multiplexing to a receiving unit through either oneof the main transmission line or the backup transmission line, and areceiving unit receiving the optical signal transmitted from the sendingunit from either one of the main transmission line or the backuptransmission line, is characterized by; the sending unit which includesa pre-emphasis section adjusting each wavelength signal level and atransmitter multiplexing each wavelength signal of which level isadjusted by the pre-emphasis section and transmitting the multiplexedwavelength signal, and a controller controlling the pre-emphasis sectionto adjust each wavelength signal level, based on control datatransmitted from the receiving unit for adjusting each wavelength signallevel; and the receiving unit includes a measurement section measuringsignal quality of each wavelength included in the received opticalsignal, a memory storing an initial value of the signal quality of eachwavelength measured by the measurement section before the transmissionline is switched over either from the main transmission line to thebackup transmission line or from the backup transmission line to themain transmission line, and a transmitter causing the measurementsection to measure signal quality of each wavelength included in theoptical signal received after the switchover when the transmission lineswitchover is performed, and generating control data based on both theinitial quality value and the post-switchover quality value, andtransmitting the generated control data to the sending unit.

The third aspect of the present invention of the sending unittransmitting optical signals multiplexed with wavelength divisionmultiplexing to a receiving unit through either one of the maintransmission line or the backup transmission line, is characterized by;the sending unit which includes a pre-emphasis section adjusting eachwavelength signal level, a transmitter multiplexing each wavelengthsignal of which level is adjusted by the pre-emphasis section andtransmitting the multiplexed wavelength signal, and a controllercontrolling the pre-emphasis section to adjust each wavelength signallevel, based on control data transmitted from the receiving unit foradjusting each wavelength signal level.

The third aspect of the present invention of the receiving unitreceiving a multiplexed optical signal transmitted from a sending unitthrough the main transmission line or the backup transmission line fromeither one of the main transmission line or the backup transmissionline, is characterized by; a measurement section measuring the signalquality of each wavelength included in the received optical signal, amemory storing an initial value of the signal quality of each wavelengthmeasured by the measurement section before the transmission line isswitched over either from the main transmission line to the backuptransmission line or from the backup transmission line to the maintransmission line, and a transmitter ordering the measurement section tomeasure the signal quality of each wavelength included in the opticalsignal received after the switchover when the transmission lineswitchover is performed, and transmitting to the sending unit controldata by which the sending unit controls a transmission level of eachwavelength, based on both the initial quality value and thepost-switchover quality value.

The third aspect of the present invention provides ability to avoiddeterioration of the reception quality of the optical signal caused bythe transmission line switchover by readjustment after the switchover.

The fourth aspect of the present invention of the sending unit sendingoptical signals multiplexed with wavelength division multiplexing, to areceiving unit through either one of the main transmission line or thebackup transmission line, is characterized by; a pre-emphasis sectionadjusting each wavelength signal level, a transmitter multiplexing eachwavelength signal of which level is adjusted by the pre-emphasis sectionand transmitting the multiplexed wavelength signal, a memory storingcontrol data which includes adjustment amount of the pre-emphasissection correspondingly to combinations of the transmission line statesindicating which of the main transmission line and the backuptransmission line is in use for the optical signal transmission withrespect to the transmission lines located between the sending unit andthe receiving unit, and a controller controlling the pre-emphasissection to adjust each wavelength signal level, based on thetransmission line states and the control data.

The fourth aspect of the present invention also provides ability toavoid deterioration of the reception quality of the optical signalcaused by the transmission line switchover by readjustment after theswitchover.

The fifth aspect of the present invention is characterized by an opticalswitch unit which includes an optical switch switching to one side ofthe two transmission lines, and thereby an optical signal is input fromthe switched transmission line, and a dummy light output sectionoutputting dummy lights on detection of the light output from theoptical switch being intercepted.

The fifth aspect of the present invention provides ability to preventgeneration of light surge caused by the transmission line switchover bysending dummy lights.

Further scopes and features of the present invention will become moreapparent by the following description of the embodiments with theaccompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram illustrating an overall configuration of awavelength division multiplex (WDM) optical communication systemaccording to one embodiment of the present invention.

FIG. 2 shows a block diagram illustrating a detailed configuration of aterminal station.

FIG. 3 shows a block diagram illustrating a detailed configuration of arelay station.

FIG. 4 shows an exemplary OSC structure.

FIG. 5 shows a sequence diagram illustrating a processing flow of dummylight transmission.

FIG. 6 shows an explanation diagram of ASE correction.

FIG. 7 shows a sequence diagram illustrating a processing flow of thefirst method of ASE correction.

FIG. 8 shows a sequence diagram illustrating a processing flow of thesecond method of ASE correction.

FIG. 9 shows a sequence diagram illustrating a processing flow of thethird method of ASE correction.

FIG. 10A shows OSW states of a terminal station and a relay station inthe forward direction by use of symbols ‘0’ and ‘1’, in case the numberof relay stations is two.

FIG. 10B shows OSW states using a tabular form.

FIGS. 11, 12A, 12B show examples of an ASE correction table of a relaystation.

FIGS. 13A to 13C show examples of an ASE correction table of a terminalstation.

FIG. 14 shows a sequence diagram illustrating a processing flow of thefirst method of pre-emphasis control.

FIG. 15 shows a sequence diagram illustrating a processing flow of thesecond method of pre-emphasis control.

FIG. 16 shows an example of an attenuation factor table.

FIG. 17 shows a block diagram illustrating an exemplary systemconfiguration when different kinds of optical fibers are used fortransmission lines in #0 system and #1 system.

FIG. 18 shows a block diagram illustrating a schematic configurationexample of a single-wavelength optical communication system havingduplicated transmission lines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the present invention is describedhereinafter referring to the charts and drawings. However, it is to benoted that the scope of the present invention is not limited to theembodiments described below, but instead embraces all equivalents to theclaims described.

<Configuration of WDM Optical Communication System>

FIG. 1 shows a block diagram illustrating an overall configuration of awavelength division multiplex (WDM) optical communication systemaccording to one embodiment of the present invention. In this WDMoptical communication system, terminal stations 1 ₁, 1 ₂, and one ormore relay stations 2 ₁-2 _(n−1) (n is an integer more than 2) areprovided as optical transmission equipment. Further, as opticaltransmission lines, this WDM optical communication system includestransmission lines F₀₁-F_(0n), F₁₁-F_(1n), R₀₁-R_(0n), and R₁₁-R_(1n)constituted of optical fibers.

Sections between neighboring stations among terminal stations 1 ₁, 1 ₂,and relay stations 2 ₁-2 _(n−1) are termed as ‘spans’. In the WDMoptical communication system shown in FIG. 1, (n) spans SP1-SPn areformed. In these spans SP1-SPn, the transmission lines F₀₁-F_(0n),F₁₁-F_(1n), R₀₁-R_(0n), and R₁₁-R_(1n) are provided for interconnectingneighboring stations.

Each terminal station 1 ₁, 1 ₂ is commonly used as terminal station onthe transmission side (hereafter simply referred to as sending terminalstation), and terminal station on the reception side (hereafter simplyreferred to as receiving terminal station). Each relay station 2 ₁-2_(n−1) amplifies an optical signal (WDM optical signal) transmitted andreceived between both terminal stations 1 ₁, 1 ₂, and relays the opticalsignal.

The transmission lines F₀₁-F_(0n) (hereafter referred to as‘transmission line F0’) and F₁₁-F_(1n) (hereafter referred to as‘transmission line F1’) are used when an optical signal transmitted fromterminal station 1 ₁ (sending terminal station) is transmitted toterminal station 1 ₂ (receiving terminal station) through relay stations2 ₁-2 _(n−1). Among these transmission lines, the transmission line F0is the main transmission line (line in use, in-use system, #0 system, orMain), and the transmission line F1 is the backup transmission line(standby line, standby system, #1 system, or Backup) for use when afailure (line break, deterioration resulting from a secular change,increased noise, etc.) occurs on any main line.

The transmission lines R₀₁-R_(0n) (hereafter referred to as‘transmission line R0’) and R₁₁-R_(1n) (hereafter referred to as‘transmission line R1’) are used when WDM optical signals transmittedfrom terminal station 1 ₂ is transmitted to terminal station 1 ₁ throughrelay stations 2 _(n−1)-2 ₁. Among these transmission lines, thetransmission line R0 is the main transmission line, and the transmissionline R1 is the backup transmission line for use when a malfunctionoccurs on any main line.

Hereafter, a direction from terminal station 1 ₁ to terminal station 1 ₂is termed as forward direction, and the other direction from terminalstation 1 ₂ to terminal station 1 ₁ is termed as reverse direction.Further, when terminal stations 1 ₁, 1 ₂ are generically referred to,they are briefly denoted as ‘terminal station 1’ and also when relaystations 2 ₁-2 _(n−1) are generically referred to, they are brieflydenoted as ‘relay station 2’. Moreover, terminal station 1 and relaystation 2 may generically be referred to as ‘station’.

Additionally, the optical fiber includes single mode fiber (SMF), zerodispersion shift fiber (DSF), non-zero dispersion shift fiber (NZDSF),dispersion compensation fiber (DCF), etc.

FIG. 2 shows a block diagram illustrating a detailed configuration ofterminal station 1 ₁. Hereafter, the configuration of terminal station 1₁ is described. However, since terminal station 1 ₂ is structured of thesame configuration, the following description is also applicable toterminal station 1 ₂.

Terminal station 1 ₁ includes variable optical attenuators (VAT) 10 ₁-10_(m), optical multiplexer 11, optical demultiplexer 12, opticalamplifier for transmission (TAMP) 13, optical amplifier for reception(RAMP) 14, WDM filters 17, 18, optical spectrum analyzers 15, 16, OSCprocessor/controller 19 (OSC: optical supervisory channel), and OSWsection 20 (OSW: optical switch).

Further, OSW section 20 includes optical switches (span switches) 21,22, WDM filters 23, 24, a dummy light generator 25, a photo diode (PD)26, and an interception/restoration detector 27.

The number (m) of VAT 10 ₁-10 _(m) is an integer more than 2, whichrepresents the number of wavelengths (i.e. the number of channels) ofthe WDM-multiplexed communication signals. For example, m is 80, 160,176, etc.

Optical signals having wavelengths λ1-λm of communication signals arerespectively input to VAT 10 ₁- 10 _(m). The optical signals ofwavelengths λ1-λm are allocated in a wavelength band having wavelengthsof 1,528 nm-1,568 nm, which is termed as C band (conventional band) VAT10 ₁-10 _(m) attenuate or amplify the input optical signals according tothe attenuation amount controlled by spectrum analyzer 15, and supplythe attenuated optical signals, or the amplified optical signals, tooptical multiplexer 11. This attenuation or amplification performed byVAT 10 ₁-10 _(m) in regard to the optical signals of respectivewavelengths is termed as pre-emphasis control, which will be describedlater in detail.

Optical multiplexer 11 is constituted of, for example, arrayed waveguidegrating (AWG), fiber grating, WDM coupler, etc. This optical multiplexer11 multiplexes the optical signals input through VAT 10 ₁-10 _(m) intowavelengths λ1-λm by WDM, and supplies the optical signal after beingmultiplexed (hereafter referred to as ‘WDM optical signal’) to TAMP 13.

TAMP 13 has, for example, an Erbium-doped fiber amplifier (EDFA), or thelike, for directly amplifying the optical signal, and an automaticcontrol circuit for controlling the output power (output electric powerand level). TAMP 13 amplifies the input WDM optical signals to opticalsignals having an output level to which ASE (amplified spontaneousemission) correction is performed, under the control of OSCprocessor/controller 19. The ASE correction will be described later indetail.

A substantial portion of the WDM optical signal amplified by TAMP 13 issupplied to OSW section 20 through WDM filter 17, and a portion of theresidual WDM optical signal is supplied to optical spectrum analyzer 15.

Optical spectrum analyzer 15 measures signal intensity (electric powerand power level) of each wavelength and an optical signal-to-noise ratio(OSNR), respectively included in the WDM optical signal supplied fromTAMP 13, and adjusts an attenuation amount of each VAT 10 ₁-10 _(m)under the control of OSC processor/controller 19.

OSC processor/controller 19 supplies WDM filter 17 with an OSC signal tobe transmitted to the neighboring relay station 2 ₁. Also, OSCprocessor/controller 19 receives from WDM filter 18 an OSC signalincluded in the WDM optical signal received from relay station 2 ₁, andperforms predetermined processing. A wavelength in the C band, differentfrom the wavelengths λ1-λm, is allocated for the OSC signal, and the OSCsignal itself is WDM multiplexed into the WDM optical signal beingoutput from TAMP 13.

Further, OSC processor/controller 19 controls OSW section 20 (includingOSW 21, 22 and interception/restoration detector 27), optical spectrumanalyzer 15, TAMP 13, RAMP 14, etc. The detailed processing and controlperformed by OSC processor/controller 19 will be described later.

WDM filter 17 multiplexes the OSC signal supplied from OSCprocessor/controller 19 with the WDM optical signal supplied from TAMP13, and supplies the multiplexed WDM optical signal to OSW 21 in OSWsection 20.

By OSC processor/controller 19, OSW 21 is switched to one of the maintransmission line F0 side (or transmission line R0 side in case ofterminal station 1 ₂) and the backup transmission line F1 side(transmission line R1 side in case of terminal station 1 ₂). The WDMoptical signal having a plurality of optical signal wavelengths beinginput from WDM filter 17 to OSW 21 is collectively output to thetransmission line on the switched side, and is transmitted to relaystation 2 ₁ (relay station 2 _(n−)in case of terminal station 1 ₂)through the transmission line on the switched side.

Meanwhile, OSW 22 in OSW section 20 is switched to one of thetransmission line on which the WDM signal (communication signal and OSCsignal) is incoming, out of the main transmission line R0 (F0) or thebackup transmission line R1 (F1).

The WDM optical signal input to OSW 22 through one of the maintransmission line R0 (F0) or the backup transmission line R1 (F1) fromrelay station 2 ₁ (2 _(n−1)) is input to WDM filter 18 through WDMfilters 24, 23.

WDM filter 18 demultiplexes the input WDM optical signal to obtain theseparate OSC signal, and supplies the separate OSC signal to OSCprocessor/controller 19, and supplies the residual communication signal(WDM optical signal) to RAMP 14.

RAMP 14 includes an EDFA, etc. and an automatic control circuit,similarly to TAMP 13, and thereby amplifies the input WDM opticalsignals to optical signals having ASE-corrected output power (level),under the control of OSC processor/controller 19. A substantial portionof the amplified WDM optical signal is supplied to optical demultiplexer12, and a portion of the residual WDM optical signal is supplied tooptical spectrum analyzer 16.

Optical spectrum analyzer 16 measures signal intensity (power) and anOSNR for each wavelength included in the WDM optical signal suppliedfrom RAMP 14, under the control of OSC processor/controller 19. Opticalspectrum analyzer 16 then supplies the measured signal intensity andOSNR to OSC processor/controller 19.

Optical demultiplexer 12 is constituted of, for example, arrayedwaveguide grating (AWG), fiber grating, WDM coupler, etc., similarly tooptical multiplexer 11. This optical demultiplexer 12 demultiplexes theinput WDM signal into optical signals each having wavelength λ1-λm, andsupplies the demultiplexed optical signal to non-illustrated equipmentin the succeeding stage.

WDM filter 24 provided in OSW section 20 supplies a portion of the WDMoptical signal input from OSW 22 to PD 26. PD 26 converts the inputoptical signal into an electric signal and supplies the convertedelectric signal to interception/restoration detector 27.

Interception/restoration detector 27 detects interception andrestoration of the WDM optical signal, based on the electric signalsupplied from PD 26. On detection of the interception,interception/restoration detector 27 supplies dummy light generator 25with a signal instructing to output a light (dummy light). Meanwhile, ondetection of the restoration, interception/restoration detector 27supplies dummy light generator 25 with a signal instructing to halt thedummy light output.

Dummy light generator 25 outputs the dummy light under the control ofinterception/restoration detector 27. The dummy light is used as lightto be transmitted in substitution for the WDM optical signal, in theevent that the WDM optical signal is intercepted. This dummy light has awavelength different from the wavelengths λ1-λm of the WDM opticalsignal, and a predetermined power value. As the wavelength of the dummylight, for example, a shorter wavelength on or a longer wavelength inthe C band is used. Preferably, the predetermined power is substantiallythe same as the power of the WDM optical signal. The dummy light outputfrom dummy light generator 25 is input to RAMP 14 through WDM filters 23and 18. The detailed transmission processing of this dummy light will bedescribed later.

FIG. 3 shows a block diagram illustrating a detailed configuration ofrelay stations 2 _(i) (i=1 to n−1). Relay station 2 _(i) includes OSWsections 30, 40; WDM filters 51-54; optical amplifiers for relay (LAMP)55, 56; and OSC processor/controllers 57, 58.

OSW section 30 includes OSW 31, 32, WDM filters 33, 34, a dummy lightgenerator 35, a PD 36 and an interception/restoration detector 37. Also,OSW section 40 includes OSW 41, 42, WDM filters 43, 44, a dummy lightgenerator 45, a PD 46 and an interception/restoration detector 47.Functions and configurations of OSW sections 30, 40 are identical tothose of the aforementioned OSW section 20, and therefore the detaileddescription is omitted here.

To OSW section 30, a WDM optical signal (communication signal and OSCsignal) is input from the neighboring terminal station 1 ₁ or theneighboring relay station 2 _(i−1) through the main transmission line F0or the backup transmission line F1.

The input optical signal is supplied to WDM filter 51, and thenseparated into the communication signal and the OSC signal. Thecommunication signal is input to LAMP 55, while the OSC signal is inputto OSC processor/controller 57.

LAMP 55 includes an EDFA, etc. and an automatic control circuit,similarly to the aforementioned TAMP 13 or RAMP 14. LAMP 55 amplifiesthe input WDM optical signal into an optical signal having output power(level) in which ASE correction (described later in detail) isperformed, under the control of OSC processor/controller 57.

The amplified WDM optical signal is multiplexed with an OSC signal byWDM filter 53. Thereafter, the multiplexed WDM optical signal issupplied to OSW section 40, and transmitted to the neighboring relaystation 2 _(i+1) or terminal station 1 ₂ through the transmission lineF0 or F1.

OSC processor/controller 57 supplies WDM filter 52 with the OSC signalto be transmitted to the neighboring relay station 2 _(i−1). OSCprocessor/controller 57 also receives from WDM filter 51 the OSC signalincluded in the WDM optical signal having been received from relaystation 2 _(i−1), and performs predetermined processing (to be describedlater in detail). Further, OSC processor/controller 57 controls OSWsection 30 (OSW 31, 32 and interception/restoration detector 37), LAMP55, 56, etc. (to be described later in detail).

Meanwhile, to OSW section 40, a WDM optical signal (communication signaland OSC signal) is input from the neighboring terminal station 1 ₂ orthe neighboring relay station 2 _(i+1), through the main transmissionline R0 or the backup transmission line R1.

The input optical signal is supplied to WDM filter 54, and separatedinto the communication signal and the OSC signal. The communicationsignal is input to LAMP 56, while the OSC signal is input to OSCprocessor/controller 58.

LAMP 56 has a configuration similar to that of the aforementioned LAMP55, and performs the similar processing. The WDM optical signalamplified by LAMP 55 is multiplexed with an OSC signal by WDM filter 52.Thereafter, the multiplexed WDM optical signal is supplied to OSWsection 30, and transmitted to the neighboring relay station 2 _(i−1) orterminal station 1 ₁, through the transmission line R0 or R1.

OSC processor/controller 58 supplies WDM filter 53 with the OSC signalto be transmitted to the neighboring relay station 2 _(i−1). OSCprocessor/controller 58 also receives from WDM filter 54 the OSC signalincluded in the WDM optical signal having been received from relaystation 2 _(i+1) and performs predetermined processing (to be describedlater in detail). Further, OSC processor/controller 58 controls OSWsection 40 (OSW 41, 42 and interception/restoration detector 47), LAMP55, 56, etc. (to be described later in detail).

FIG. 4 shows an example of the OSC structure. OSC has fields includingcommunication line for supervision (DCC), order wire line (OW),information of the number of wavelengths, ASE correction information,switch state (SW state), control byte, and plural sets of pre-emphasisinformation.

The ‘information of the number of wavelengths’ represents the number ofchannels (namely, the number of wavelengths) multiplexed into thecommunication signal being transmitted, by use of a vector notation. Asfor this information of the number of wavelengths, OSCprocessor/controller 19 (refer to FIG. 2) of the sending terminalstation 1 writes the number of channels being multiplexed into thewavelength information field, and sends to the neighboring relay station2 located on the downstream side. Relay station 2 extracts thisinformation of the number of wavelengths from the OSC once, and rewritesthis information in the field of information of the number ofwavelengths in the OSC, and then transmits this information to relaystation 2 located on the downstream side. This procedure is repeated ineach station, and finally OSC processor/controller 19 in the receivingterminal station 1 extracts this information of the number ofwavelengths from the OSC.

In the ‘ASE correction information field’, ASE correction amount, whichwill be described later, variation of the optical signal power input toLAMP or RAMP, etc. are stored.

In the ‘SW state field’, each OSW state of terminal stations 1 ₁, 1 ₂,and relay stations 2 ₁-2 _(n−1) is stored. The OSW state is representedby ‘0’ when the corresponding OSW is switched to the main transmissionline side, and represented by ‘1’ when the corresponding OSW is switchedto the backup transmission line side.

The ‘control byte field’ includes a command part and a SW switchoverrequest indication part. As command set in the command part, code forpreceding notification of switchover, code for triggering switchover,code for ASE correction completion notification, code for pre-emphasiscontrol request, etc. are stored. Signification of these codes will bedescribed later.

The ‘pre-emphasis information fields’ are prepared for the number of,channels. In the fields, pre-emphasis control for each channel, namelyeach data for instructing an increase/decrease of each attenuationamount of VAT 10 ₁-10 _(m) is stored.

The OSC signal is terminated in each station. More specifically, the OSCsignal is multiplexed to the WDM optical signal in the OSCprocessor/controller of each station, and extracted from the WDM opticalsignal in the OSC processor/controller of the succeeding station. A newOSC signal is then multiplexed into a WDM optical signal by the OSCprocessor/controller of the succeeding station of interest, and istransmitted to the further succeeding station.

<Dummy Light Transmission Processing>

While switchover is executed when OSW is switched from the maintransmission line side (hereafter referred to as #0 system) to thebackup transmission line side (hereafter referred to as #1 system), orfrom #1 system to #0 system, an optical signal is intercepted in theOSW. As a result, the optical signal (WDM optical signal) is nottransmitted on the transmission line between the OSW. Namely, theoptical signal is intercepted (light interception) while the switchoveris executed. This causes no optical signal input to each opticalamplifier on the receiving side (LAMP 55 or 56 in relay station 2, andRAMP 14 in terminal station 1) during the interception.

After completion of the switchover, optical signal output to thetransmission line from the OSW is resumed, and accordingly the opticalsignal is again input to the optical amplifier.

As such, when the light to be input to the optical amplifier isintercepted once, and input to the optical amplifier is resumed afterthe interception, the optical amplifier may possibly emit the energyhaving been accumulated after the interception immediately, togetherwith the optical signal after being amplified from the input opticalsignal. In other words, a light surge may possibly occur. This lightsurge may damage (for example, melt) the transmission line (opticalfiber).

In particular, since the optical amplifier amplifying the WDM opticalsignal outputs an optical signal in which power of the plural channelsis added, the output optical signal power itself is large, andtherefore, highly possibly, superposition of the light surge onto thispower may damage the transmission line.

In order to prevent such occurrence of the light surge and to protectthe transmission line, according to the embodiment of the presentinvention, the dummy light is input to the optical amplifiers (LAMP 55,56 and RAMP 14) during the switchover.

This dummy light transmission is executed by interception/restorationdetector 27 (37, 47) and dummy light generator 25 (35, 45), as describedearlier (refer to FIGS. 2, 3).

FIG. 5 shows a sequence diagram illustrating a processing flow of thedummy light transmission. This sequence diagram illustrates, as anexample, the transmission line between terminal station 1 ₁ (station onthe upstream side, hereafter referred to as upstream station) and relaystation 2 ₁ (station on the downstream side, hereafter referred to asdownstream station) is switched over from the transmission line F0 tothe transmission line F1. Namely, both OSW 21 (refer to FIG. 2) in OSWsection 20 and OSW 32 (refer to FIG. 3) in OSW section 30 are switchedover from #0 system to #1 system.

First, when the switchover of OSW 21 is to start, OSCprocessor/controller 19 in the upstream station transmits a precedingnotification of switch over (more specifically, a code for the precedingnotification of switchover, which is written in the command part of thecontrol byte field in OSC (refer to FIG. 5)) to the neighboringdownstream station, so as to notify OSC processor/controller 57 in thedownstream station in advance, that the switchover is to be started(step S1).

The code for the preceding notification of switchover includesidentification information of the station in which the switchover is tobe executed; OSW identification information indicative of whether OSW 21or OSW 22 is to be switched; and the direction of switchover indicativeof whether the switchover is to be performed from #0 system to #1system, or from #1 system to #0 system. In the example shown here, theidentification information of terminal station 1 ₁, and theidentification information of OSW 21, and the switchover direction from#0 system to #1 system are included in the code for the precedingnotification of switchover. From the above information, OSCprocessor/controller 57 in the downstream station recognizes that OSW 32corresponding to OSW 21 is to be switched.

Subsequently, OSC processor/controller 19 in the upstream stationtransmits a switchover trigger signal (code for triggering switchoverwritten in the command part of the OSC control byte) to OSCprocessor/controller 57 in the downstream station (step S2). Thisswitchover trigger signal aims at reducing a time lag between theswitchover timing of OSW 52 in the upstream station and the switchovertiming of OSW 32 in the downstream station.

By sending and receiving the switchover trigger signal, OSCprocessor/controller 19 in the upstream station switches OSW 21 from #0system to #1 system, and also OSC processor/controller 57 in thedownstream station switches OSW 32 from #0 system to #1 system (stepsS3, S4). By these switchovers, the transmission line in the directionfrom the upstream station to the downstream station is switched overfrom the main transmission line F0 to the backup transmission line F1.

During the period from the start to the completion of the switchover,the output light of OSW 21 and the output light of OSW 32 areintercepted. As a result, the light to be input to PD 36 in OSW section30 of the downstream station is also intercepted, and the electricsignal power (level) input to interception/restoration detector 37 fromPD 36 is decreased.

OSC processor/controller 57 in the downstream station supplies a value(m), denoting the number of channels, to interception/restorationdetector 37 in advance, based on the information of the number ofwavelengths included in the received OSC. Interception/restorationdetector 37 stores the supplied number of channels (m) into an internalstorage (semiconductor memory, or the like). Also,interception/restoration detector 37 maintains, in the internal storage,a threshold (power value) to determine whether one channel signal (onewavelength) is in transmission.

Interception/restoration detector 37 divides the electric signal powerfrom PD 36 by the stored number of channels (m), so as to obtainelectric signal power value per channel. Interception/restorationdetector 37 then compares this power value per channel with the storedthreshold. If the former is smaller than the latter,interception/restoration detector 37 determines that the optical signalis intercepted (namely, the optical signal is not transmitted). On theother hand, if the former is not smaller than the latter,interception/restoration detector 37 determines that the optical signalis not intercepted (namely, the optical signal is being transmitted, orhas been restored).

On determining that the optical signal is intercepted,interception/restoration detector 37 instructs dummy light generator 35to output a dummy light. With this, dummy light generator 35 outputs thedummy light to WDM filter 33 (step S5). The dummy light is input to LAMP55 through WDM filters 33, 51. As a result, occurrence of aninterception condition of the input light to LAMP 55 can be prevented.

LAMP 55 amplifies this dummy light and outputs the amplified light.Accordingly, the dummy light is also input into the optical amplifiersprovided in the stages succeeding to LAMP 55 (LAMP 55 in relay station 2of the succeeding stage, and RAMP 14 in the receiving terminal station1). Thus, occurrence of an interception condition of the light input tothe optical amplifiers in the succeeding stages can be prevented.

Meanwhile, after the interception, when the switchovers of OSW 21, 32are completed, and the WDM optical signal output from OSW 32 isrestored, electric signal power being output from PD 36 is increased. Asa result, interception/restoration detector 37 determines that theoptical signal is not intercepted any more (namely, the optical signalhas been restored). With this determination, dummy light generator 35halts outputting the dummy light (step S6). As a result, only the WDMoptical signal is input to LAMP 55.

As such, even in a condition that the WDM optical signal is not input toLAMP 55 from OSW 32, the dummy light is input to LAMP 55 and the opticalamplifiers in the succeeding stages (on the downstream side), andaccordingly, generation of the light surge in the optical amplifiers isprevented.

Further, since the dummy light is output only when the WDM opticalsignal is intercepted, a cost increase of electric power, etc. can berestrained.

Additionally, the preceding notification of switchover and theswitchover trigger signal may be transmitted from the downstream stationto the upstream station.

Also, in the above description, the switchover of the transmission linein the forward direction between terminal station 1 ₁ and relay station2 has been explained. As to the dummy light transmission processing atthe time of switching the transmission line in the reverse direction,the same processing as the aforementioned processing is performed,except for replacing the upstream station with the downstream station.For example, interception/restoration detector 27 of terminal station 1₁ decides an interception/restoration of the optical signal based on theelectric signal from PD 26, and according to this decision,interception/restoration detector 27 controls dummy light generator 25to output/halt the dummy light. Also, the dummy light transmissionprocessing is performed in a similar manner in the cases of atransmission line switchover between relay station 2 _(i) and relaystation 2 _(i+1) in the forward direction, a transmission lineswitchover therebetween in the reverse direction, a transmission lineswitchover between relay station 2 _(n−1) and terminal station 1 ₂ inthe forward direction, and a transmission line switchover therebetweenin the reverse direction.

<ASE Correction>

When a transmission loss varies as a result of an OSW switchover, theinput power (input level) being input to an optical amplifier alsovaries. As the input level varies, the ASE light level generated by theoptical amplifier varies, which may influence amplification in thesucceeding amplifiers. Such an undesirable influence can be solved byASE correction.

FIG. 6 shows an explanation diagram of the ASE correction. For the sakeof easy understanding of the explanation, a block diagram is illustratedonly for the portions including the optical amplifiers in both a sendingterminal station 1 ₁ and relay stations 2 ₁, 2 ₂ on the downstream side(TAMP 13 and two LAMP 55), OSW disposed between these opticalamplifiers, and transmission lines.

An optical amplifier A1 shown in the preceding stage corresponds to TAMP13 in terminal station 1 ₁, an optical amplifier A2 shown in the middlestage corresponds to LAMP 55 in relay station 2 ₁, and an opticalamplifier A3 shown in the succeeding stage corresponds to LAMP 55 inrelay station 2 ₂, respectively.

Each optical amplifier A1-A3 (and any other optical amplifiers includedin the other relay stations and the terminal station) executes automaticlevel control (ALC) This ALC signifies that the optical amplifiermonitors both optical signal power (input level) which is input to theoptical amplifier concerned (unit: [W] or [mW]) and optical signal power(output level) which is output from the optical amplifier concerned(unit: [W] or [mW]), and controls an amplification factor (which isreferred to as ‘amplification amount’ in case of the dB notation) sothat the output level reaches a target level.

A communication signal input to the optical amplifier A1 is amplified inthe optical amplifier A1, and the amplified signal is output therefrom.Assuming that the amplification factor of the optical amplifier A1 isG1, the power (level) of the communication signal for one channel isPin0, the number of multiplexed wavelengths is m, the output level isPout1, then the output level Pout1=G1·m·Pin0, when no noise component isincluded. Accordingly, the optical amplifier A1 executes ALC by settingthe target level to G1·m·Pin0.

However, when the optical amplifier A1 amplifies the input opticalsignal and outputs the amplified signal, an ASE (amplified spontaneousemission) light (noise component) is simultaneously output. Because ofthis ASE light, the output level Pout1 becomes as follows:$\begin{matrix}\begin{matrix}{{{Pout}\quad 1} = {{G\quad{1 \cdot m \cdot {Pin}}\quad 0} + {{Pase}\quad 1}}} \\{= {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1}}}\end{matrix} & (1)\end{matrix}$

Here, Pase1 is an ASE light (noise component) level (ASE level). Also,Pin1 is a total level, m·Pin0, of (m) input optical signals.

Therefore, it is required that the optical amplifier A1 executes ALCwith the target level set to (G1·Pin1+Pase1).

Here, in order that the optical signal can be received without producingerrors on the receiving side even if the optical signal is attenuated inthe transmission line, the communication signal level (signal level)G1·Pin1 included in the output light is required to have a predeterminedlevel (shown as S). Namely,G1·Pin1=S   (2)

Further, the predetermined level S equals to a predetermined level(defined as Psig) of the optical signal per channel multiplied by thenumber of the multiplexed channels (m). Namely,S=m·Psig   (3)

In the optical amplifier A1 (and other optical amplifiers), thepredetermined level (Psig) per channel is stored in advance, and thepredetermined value S is obtained by multiplying Psig by the number ofmultiplexed channels (m).

From the above formulae (1)-(3), the following is obtained:$\begin{matrix}\begin{matrix}{{{Pout}\quad 1} = {S + {{Pase}\quad 1}}} \\{= {{m \cdot {Psig}} + {{Pase}\quad 1}}}\end{matrix} & (4)\end{matrix}$

Meanwhile, the ASE level Pase1 can be expressed as follows:Pase1=h·νPin1·( G1−1)·B·Nf   (5)where, h is Planck constant, ν is an ASE light frequency, B is a gainbandwidth of the optical amplifier A1. Also, Nf is a noise figure of theoptical amplifier A1, which varies with the input level Pin1. However,Nf can be obtained based on the input level Pin1 (for example, using acalculation formula, or a table of correspondence between Pin1 and Nf).

The optical amplifier A1 (and the other optical amplifiers) retains theformula (5), and the calculation formula or the table of correspondencefor obtaining Nf. Thus, the own ASE level Pase1 can be obtained. (Thesame method is applicable to other optical amplifiers.)

Therefore, the optical amplifier A1 obtains the levels m·Psig and Pase1,obtains the target level Pout1 therefrom, using the formula (4), andthen executes ALC so that the monitored output level reaches equal tothe obtained target level Pout1. With this, it becomes possible to setthe communication signal level included in the output light of theoptical amplifier A1 equal to the predetermined level S.

As such, correcting the target level to have a value in which the ASElevel is added to the signal level S=G1·Pin1, namely (G1·pin1+Pase1), istermed as ASE correction. And,η1=(G1·Pin1+Pase1)/(G1·Pin1)   (6)is defined as ASE correction factor.

Additionally, the optical amplifier A1 can obtain the target level bymultiplying the predetermined level S (=G1·Pin1) by the ASE correctionfactor η1.

The optical signal output from the optical amplifier A1 is input to theoptical amplifier A2 through OSW 21, the main transmission line F0, andOSW 32. At this time, the optical signal attenuates to some extent (withan attenuation factor L1) by the main transmission line F0.

Therefore, assuming an optical signal level of input to the opticalamplifier A2 is Pin2, $\begin{matrix}\begin{matrix}{{{Pin}\quad 2} = {{Pout}\quad{1/L}\quad 1}} \\{= {{\left( {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1}} \right)/L}\quad 1}}\end{matrix} & (7)\end{matrix}$

Assuming an amplification factor of the optical amplifier A2 is G2, atarget level (output level) is Pout2, and an ASE level is Pase2,$\begin{matrix}\begin{matrix}{{{Pout}\quad 2} = {{G\quad{2 \cdot {Pin}}\quad 2} + {{Pase}\quad 2}}} \\{= {{G\quad{2 \cdot {\left( {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1}} \right)/L}}\quad 1} + {{Pase}\quad 2}}} \\{= {{G\quad{2 \cdot G}\quad{1 \cdot {Pin}}\quad{1/L}\quad 1} + {G\quad{2 \cdot {Pase}}\quad{1/L}\quad 1} + {{Pase}\quad 2}}}\end{matrix} & (8)\end{matrix}$

In the optical amplifier A2 also, it is required that the communicationsignal level included in the output light reaches the predeterminedlevel S (=G1·Pin1). In the above-mentioned formula (8), since the signallevel is G2·G1·Pin1/L1,S=G1·Pin1=G2·G1·Pin1/L1   (9)Namely, it is sufficient if G2=L1. Therefore,Pout2=G1·Pin1+Pase1+Pase2   (10)

Accordingly, the optical amplifier A2 monitors the output level, andexecutes ALC so that the output level reaches the value (target level)shown in the formula (10).

Here, in the optical amplifier A2, the value of Psig is stored inadvance, as in the case of the optical amplifier A1. Thus, the opticalamplifier A2 can obtain the predetermined level S=G1·Pin1 by multiplyingPsig by the number of channels (m) supplied from the information of thenumber of wavelengths in OSC from the upstream station (terminal station1 ₁ in this case). Also, the optical amplifier A2 can obtain the ASElevel, Pase2, output from the optical amplifier A2, according to theabove-mentioned formula (5).

On the other hand, since the ASE level Pase1 is the value for theoptical amplifier A1 in the preceding stage, the optical amplifier A2cannot acquire this Pase1. Therefore, by receiving the ASE level, Pase1,of the optical amplifier A1 in the preceding stage from this opticalamplifier A1, the optical amplifier A2 obtains the target levelaccording to the formula (10).

Additionally, assuming the ASE correction factor of the opticalamplifier A2 is η2, $\begin{matrix}\begin{matrix}{{\eta\quad 2} = {\left( {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1} + {{Pase}\quad 2}} \right)/\left( {G\quad{1 \cdot {Pin}}\quad 1} \right)}} \\{= {{\eta\quad 1} + {{Pase}\quad{2/\left( {G\quad{1 \cdot {Pin}}\quad 1} \right)}}}}\end{matrix} & (11)\end{matrix}$Thus, it is also possible for the optical amplifier A2 to obtain thetarget level by multiplying the predetermined level S (=G1·Pin1) by theASE correction factor η2.

In a similar way, as to the optical amplifier A3, the target level(output level) Pout3 becomes, $\begin{matrix}\begin{matrix}{{{Pout}\quad 3} = {{G\quad{3 \cdot {\left( {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1} + {{Pase}\quad 2}} \right)/L}}\quad 2} + {{Pase}\quad 3}}} \\{= {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1} + {{Pase}\quad 2} + {{Pase}\quad 3}}}\end{matrix} & (12)\end{matrix}$where, G3 is the amplification factor of the optical amplifier A3, Pase3is the ASE level of the optical amplifier A3, and L2 is the attenuationfactor of the transmission line from the optical amplifier A2 to theoptical amplifier A3.

The optical amplifier A3 can acquire the target level Pout3 by receivingboth the ASE level Pase1 of the optical amplifier A1 and the ASE levelPase2 of the optical amplifier A2 (namely Pase1+Pase2) from the opticalamplifier A2 disposed in the middle stage. With this, the opticalamplifier A3 executes ALC.

Additionally, an ASE correction factor η3 becomes, $\begin{matrix}\begin{matrix}{{\eta\quad 3} = {\left( {{G\quad{1 \cdot {Pin}}\quad 1} + {{Pase}\quad 1} + {{Pase}\quad 2} + {{Pase}\quad 3}} \right)/\left( {G\quad{1 \cdot {Pin}}\quad 1} \right)}} \\{= {{\eta\quad 2} + {{Pase}\quad{3/\left( {G\quad{1 \cdot {Pin}}\quad 1} \right)}}}}\end{matrix} & (13)\end{matrix}$

As such, each optical amplifier receives, from the optical amplifierlocated on the upstream side (in the preceding stage), the total value(hereafter referred to as ‘ASE correction amount’) of the ASE levelsaccumulated in the entire optical amplifiers located on the upstreamside. Thus, each target level of the output signal from each opticalamplifier can be obtained, and based on the obtained target level, eachoptical amplifier executes ALC. The ASE correction amount is expressedby the following formula, on assumption that the number of opticalamplifiers located on the upstream side of the optical amplifier ofinterest is q, the ASE level of the i-th optical amplifier is Pasei:$\begin{matrix}{C = {\sum\limits_{i = 1}^{q}{{Pase}\quad i}}} & (14)\end{matrix}$

The output level of the optical amplifier A1 in the preceding stage iscontrolled so as to become the target level as a result of ALC executedby the optical amplifier A1 in the preceding stage. Therefore, so far asthe attenuation amount L1 of the transmission line F0 is not varied, theinput level of the optical signal input to the optical amplifier A2 inthe middle stage has a constant value. As a result, the ASE level Pase2output from the optical amplifier A2 in the middle stage has a constantvalue.

Accordingly, once the optical amplifier A2 in the middle stage suppliesthe own ASE level Pase2 (and the ASE level Pase1 of the opticalamplifier A1 in the preceding stage) to the optical amplifier A3 in thesucceeding stage, the optical amplifier A3 in the succeeding stage canperform the ASE correction (ALC) based on the same ASE correction amount(Pase1+Pase2).

However, when the transmission line is switched over from #0 system to#1 system, or from #1 system #0 system, as a result of the switchoversof OSW 21, 32 disposed between the optical amplifiers A1, A2, theattenuation factor L1 is varied as a result of variations oftransmission line lengths, quality of the material of the transmissionline, transmission line parameters, etc. caused by secular change, orthe like. As a result, as it is apparent from the above formula (7), theinput level Pin2 of the optical amplifier A2 in the middle stage isvaried (either increases or decreases). Also, apparently from the aboveformula (5), the ASE level Pase2 of the optical amplifier A2 is varied.

When the ASE level Pase2 of the optical amplifier A2 is varied, thisinfluences ALC (ASE correction) of the optical amplifier A3 in thesucceeding stage, and the other optical amplifier(s) in the furthersucceeding stage(s), as is apparent from the formula (12).

Therefore, even on the occurrence of switchover, ALC and the ASEcorrection have to be performed properly so that the signal levelincluded in the optical signal output from each optical amplifier hasthe predetermined value S. As to methods for the ASE correction, thefollowing three methods are considered.

(1) First Method of ASE Correction

The first method of ASE correction is that, on the occurrence of an OSWswitchover, a downstream station transmits a new ASE correction amountto the neighboring downstream station, and that the downstream stationperforms the ASE correction based on the newly received ASE correctionamount.

FIG. 7 shows a sequence diagram illustrating a processing flow of thefirst method of the ASE correction. This sequence diagram illustrates,as one example, a case that the transmission line between terminalstation 1 ₁ (upstream station) and relay station 2 ₁ (downstream stationN1) is switched from #0 system to #1 system (refer to FIG. 1). Here, thesame symbols are assigned to the same processing as that having beendescribed in FIG. 5, and the detailed description of the processing isomitted.

With the transmission of a preceding notification of switchover and aswitchover trigger signal from the upstream station (steps S1, S2), bothOSW 21 in the upstream station and OSW 32 in the downstream station N1are switched from #0 system to #1 system, and the transmission linebetween these stations is switched from the transmission line F0 to thetransmission line F1 (refer to FIGS. 2, 3, which are also referred to inthe following.)

Additionally, while this switchover is in execution, the downstreamstation N1 performs the aforementioned dummy light transmission.Further, it may also be possible that the downstream stations N1-Nn-1(relay station 2 ₁-2 _(n−1)) successively transmit the precedingnotification of switchover to each neighboring downstream station N2-Nn(relay station 2 ₂-2 _(n−1) , terminal station 1 ₂) (steps S14, S15). Insuch a case, OSC processor/controller 58 in each relay station 2 ₁-2_(n−1) stores the preceding notification of switchover in the commandpart of the control byte of OSC, in the form of a code for the precedingnotification of switchover, and transmits the notification through WDMfilter 53.

After the switchover is completed, LAMP 55 (optical amplifier A2 in FIG.6) in the downstream station N1 calculates the own ASE level Pase2according to the above formula (5), based on the input level Pin1 beinginput after the switchover. An ASE correction amount (Pase1+Pase2) isthen obtained using the own ASE level Pase2 calculated above and the ASEcorrection amount Pase1 supplied from the upstream station. Thereafter,LAMP 55 acquires the target level based on the obtained ASE correctionamount, and executes ALC according to this target level (step S11).

Here, the ASE correction amount Pase1 of the upstream station has beensupplied, as initial value, to LAMP 55 of the downstream station at thetime of initiating this WDM optical communication system, and stored inLAMP 55.

LAMP 55 transmits the WDM optical signal, which is controlled by ALC tothe target level, to the downstream station N2. At the same time, LAMP55 supplies the newly obtained ASE correction amount (Pase1+Pase2) toOSC processor/controller 58.

OSC processor/controller 58 writes the supplied ASE correction amount(Pase1+Pase2) into the ASE correction information field in OSC, and alsowrites a code for a notification of the ASE correction completion intothe control byte field (command part) of OSC. OSC processor/controller58 then transmits this OSC signal to the downstream station N2 throughWDM filter 53.

The OSC signal transmitted from the downstream station N1 is supplied toOSC processor/controller 57 in the downstream station N2 through WDMfilter 51. OSC processor/controller 57 in the downstream station N2recognizes that the ASE correction amount has been varied, by receivinga code for the notification of the ASE correction completion beingincluded in the control byte field (command part) of OSC. With this, OSCprocessor/controller 57 in the downstream station N2 reads out the ASEcorrection amount (Pase1+Pase2) from the ASE correction informationfield in OSC, and supplies this ASE correction amount to LAMP 55 in thedownstream station N2.

LAMP 55 in the downstream station N2 executes the ASE correction andALC, similarly to the aforementioned LAMP 55 in the downstream stationN1, and supplies the ASE correction amount (Pase1+Pase2+Pase3) havingbeen obtained by the processing of the ASE correction and ALC, to OSCprocessor/controller 58. This ASE correction amount is furthertransmitted to the downstream station N3.

Such processing is successively performed in each downstream station.

Sufficiently, this transmission of the ASE correction amount to thedownstream station is performed only once immediately after thetransmission line switchover. The reason is that each downstream stationN1-Nn stores each new ASE correction amount transmitted from thepreceding station, and thereafter the ASE correction can be performedbased on the stored new ASE correction amount. Also, by limiting thetransmission to only once after the switchover, processing cost in eachstation can be reduced.

The same processing as shown above is performed when transmission lineswitchover is performed in any other transmission lines.

And, thereafter, when any of the transmission line is switched, new ASEcorrection amount is acquired in the downstream station located in thedownstream point of the switchover, and the acquired ASE correctionamount is transmitted between the stations concerned.

According to this first method, even when the loss, or the like, isvaried because of the switchover, this variation is compensated,enabling proper performance of the ASE correction against the opticalamplifier in each station. As a result, ALC can be executed so thatsignal levels can be maintained to the predetermined levels. Further,this first method does not require any optical attenuator on thetransmission line, and is applicable to networks of any structure.

Additionally, it may also be possible for the downstream station N1 tomonitor whether the input level is varied when the switchover has beenperformed. If the input level is not varied (namely, if the sameattenuation factor (loss) of the transmission line is maintained beforeand after the switchover), the processing shown in FIG. 7 can beomitted, enabling processing cost reduction.

Also, the ASE correction amount may be another value than that shown inthe formula (13), if it is possible to obtain an accumulated value ofthe ASE level accumulated from the optical amplifier in the first stageto the optical amplifier in the preceding stage. For example, the valuesindicative of the output level (target level) of the preceding opticalamplifier, the ratio of the ASE level to the output level, the ratio ofthe signal level to the output level, etc. may be supplied to thesucceeding optical amplifier, as ASE correction amount.

Further, processing shown in the steps S1, S2 may be executed by thedownstream station N1, and the preceding notification of switchover andthe switchover trigger signal may be transmitted from the downstreamstation N1 toward the upstream stations. (This is also applicable inboth the second method and the third method described in the following.)

(2) Second Method of ASE Correction

The second method of ASE correction is that the upstream stationcorrects the output level on the by OSW switchover.

FIG. 8 shows a sequence diagram illustrating a processing flow of thesecond method of the ASE correction. This sequence diagram illustrates,as one example, a case that the transmission line between terminalstation 1 ₁ (upstream station) and relay station 2 ₁ (downstream stationN1) is switched from #0 system to #1 system. The same numerals areassigned to the same processing as that having been described in FIG. 5or FIG. 7, and the detailed description of the processing is omitted.

By transmitting a preceding notification of switchover and a switchovertrigger signal from the upstream station (steps S1, S2), OSW 21 in theupstream station and OSW 32 in the downstream station N1 is switchedfrom #0 system to #1 system, and the transmission line between thesestations is switched from the transmission line F0 to the transmissionline F1. Here, while this switchover is in execution, the downstreamstation N1 performs the aforementioned dummy light transmission.

After the switchover is completed, LAMP 55 in a downstream station N2measures the input level (reception level). LAMP 55 then acquires a rateof change ΔX of the input Pin21 after the switchover, to the input levelPin20 before the switchover.ΔX=Pin21/Pin20   (14)

This rate of change Δx corresponds to the rate of change ΔL of thetransmission line attenuation factor caused by the transmission lineswitchover. Namely, when the attenuation factor of the transmission lineis varied from L10 to L11, it is possible to define ΔL as follows:ΔL=L11/L10=1/ΔX=Pin20/Pin21   (15)

Therefore, in the optical amplifier (TAMP 13) of the upstream station,by producing the output level thereof after the switchover ΔL times(=1/ΔX times) as high as the output level before the switchover, thedownstream station N1 can receive the WDM optical signal having the samelevel as that received before the switchover.

Namely, LAMP 55 supplies this rate of change ΔX to OSCprocessor/controller 57, and OSC processor/controller 57 writes the rateof change ΔX into the ASE correction information field of OSC, andtransmits this rate of change ΔX to the upstream station (step S21).

OSC processor/controller 19 in the upstream station supplies the rate ofchange ΔX received from the downstream station N1 to TAMP 13. TAMP 13executes ALC with a new target level obtained by multiplying the targetlevel having been used up to the present by 1/ΔX.

In each optical amplifier provided in the downstream station N1 and thedownstream stations N2-Nn, because the ASE correction amount is notvaried, the ASE correction is performed in the same way as that havingbeen performed so far, based on the ASE correction amount having beenstored.

The similar processing is performed when such a switchover is performedon any other transmission lines.

In addition, the aforementioned modification of the target level(namely, modification of output level) in TAMP 13 of the upstreamstation is preferably maintained within a range such that, after themodification of the output level, the ratio of the signal level (levelof the communication signal component) to the ASE level included in theoutput signal after the modification substantially equals to the ratioof the signal level to the ASE level included in the output signalbefore the modification.

According to this second method also, even when the loss, or the like,is varied resulting from the switchover, this variation is compensated,and the optical amplifier in each station can perform the ASE correctionproperly. As a result, ALC can be executed so that the signal level canbe maintained to the predetermined level S. Further, this second methodrequires no optical attenuator on the transmission lines, and isapplicable to networks of any structure. Also, according to the secondmethod, only two stations having performed the switchover perform theprocessing, without need of processing in other stations. Therefore, theprocessing cost can be reduced.

Additionally, in case that the input level to LAMP 55 of the downstreamstation N1 is not varied (namely, the same attenuation factor of thetransmission line is maintained before and after the switchover), it mayalso be possible for the downstream station N1 not to transmit the rateof change ΔX to the upstream station. With this, processing cost can bereduced. Further, the rate of change ΔX is merely an example, and othervalues (for example, the reciprocal (=1/ΔX)) may be applicable.

(3) Third Method of ASE Correction

The third method of the ASE correction is that each station retains anASE correction table specifying an ASE correction factor η correspondingto each condition of whether the optical signal is being transmittedusing #0 system or #1 system between each station (in other words, OSWstate of each station, i.e. either #0 system or #1 system), and thateach station performs the ASE correction and ALC based on this ASEcorrection table.

FIG. 10A shows OSW states in terminal station 1 and relay stations 2 ₁,2 ₂ in the forward direction using symbols ‘0’ and ‘1’, in case thenumber of relay stations is two (namely, n=3 in FIG. 1). The symbol ‘0’denotes a state that the OSW is switched to a main system (namely, #0system), while the symbol ‘1’ denotes a state that the OSW is switchedto a backup system (namely, #1 system).

In order to transmit optical signals between two stations, OSW state inboth ends of the span has to be coincident. For example, as to a spanSP1, when OSW 21 in terminal station 1 ₁ is switched to #0 system, alsoOSW 32 on the receiving side of relay station 2 ₁ has to be switched to#0 system. Also, when OSW 21 in terminal station 1 ₁ is switched to #1system, OSW 32 on the receiving side of relay station 2 ₁ has to beswitched to #1 system. Namely, the OSW state on the sending side has tobe coincident with the OSW state on the receiving side. Therefore, thenumber of the OSW states specifying the states of three spans SP1-SP3(either #0 system or #1 system) located between the successive fourstations becomes 2³=8. (In general, there are 2^(n) OSW states when thenumber of spans is n.) FIG. 10B denotes such OSW states shown in atabular form.

With respect to these eight (8) combinations of OSW states, tablesspecifying each predetermined ASE correction factor η are provided inrelay stations 2 ₁, 2 ₂ and the receiving terminal station 1 ₂.

FIG. 11 shows an example of an ASE correction table in relay station 2₁. On the upstream side of relay station 2 ₁ in the forward direction,only the sending terminal station 1 ₁ exists. Therefore, relay station 2₁ includes only two ASE correction factors, corresponding to the statesof OSW 21 in the sending terminal station 1 ₁, and OSW 32 of the relaystation 2 ₁ concerned. When OSW 21 and OSW 32 are switched on #0 system,an ASE correction factor η10 is selected, while when OSW 21 and OSW 32are switched on #1 system, an ASE correction factor η11 is selected.According to the selected correction factor, a target level isdetermined.

FIGS. 12A, 12B show examples of the ASE correction table in relaystation 2 ₂. On the upstream side of relay station 2 ₂ in the forwarddirection, the sending terminal station 1 ₁ and relay station 2 ₁ exist.Therefore, there are provided an ASE correction table 1 (refer to FIG.12A) for use on the occurrence of switchover between the sendingterminal station 1 ₁ and relay station 2 ₁, and an ASE correction table2 (refer to FIG. 12B) for use on the occurrence of switchover betweenrelay station 2 ₁ and relay station 2 ₂.

ASE correction table 1 includes four (4) patterns of the ASE correctionfactors, corresponding to OSW 21 in the sending terminal station 1 ₁,OSW 32 and OSW 41 in relay station 2 ₁, and OSW 32 in relay station 2 ₂.By way of example, if both OSW 21 in the sending terminal station 1 ₁and OSW 32 in relay station 2 ₁ are switched onto #0 system, and bothOSW 41 of relay station 2 ₁ and OSW 32 of relay station 2 ₂ are switchedonto #1 system, then an ASE correction factor η21 is selected.

ASE correction table 2 includes two kinds of the ASE correction factors,corresponding to OSW 41 of relay station 2 ₁ and OSW 32 of relay station2 ₂.

FIGS. 13A-13C show examples of the ASE correction table of terminalstation 1 ₂. On the upstream side of terminal station 1 ₂ in the forwarddirection, the sending terminal station 1 ₁ and relay stations 2 ₁, 2 ₂exists. Accordingly, there are provided an ASE correction table 11(refer to FIG. 13A) for use on the occurrence of switch over between thesending terminal station 1 ₁ and relay station 2 ₁, and an ASEcorrection table 12 (refer to FIG. 13B) for use on the occurrence ofswitchover between relay station 2 ₁ and relay station 2 ₂, and an ASEcorrection table 13 (refer to FIG. 13C) for use on the occurrence ofswitchover between relay station 2 ₂ and terminal station 1 ₂.

ASE correction table 11 has 8 kinds of the ASE correction factors, ASEcorrection table 12 has 4 kinds of the ASE correction factors, and ASEcorrection table 13 has 2 kinds of the ASE correction factors.

These ASE correction factors are obtained from the above formulae (6),(11), etc., based on the input level of each optical amplifier, the ASEcorrection amount, etc. which are measured through experiments, systemtest operation, actual system operation, etc. These ASE correctionfactors are stored in the optical amplifiers (LAMP 55, 56, RAMP 14) orOSC processor/controllers 19, 57, 58 provided in the respectivestations.

FIG. 9 shows a sequence diagram illustrating a processing flow of thethird method of the ASE correction. In this sequence diagram, as anexample, the processing performed when the transmission line in usebetween terminal station 1 ₁ (upstream station) and relay station 2 ₁(downstream station N1) is switched from #0 system to #1 system. Thesame symbols are assigned to the same processing as that shown in FIG.5, and the detailed description is omitted.

The upstream station transmits a preceding notification of switchoverand a switch state to the downstream station N1 using OSC (step S31).The preceding notification of switchover is written into the controlbyte field (command part) in OSC, and a switch state is written in theSW state field in OSC (refer to FIG. 4).

As described earlier, the preceding notification of switchover includesidentification information of the station executing the switchover, OSWidentification information, and a direction of the switchover. The SWstate includes OSW identification information and an OSW state. The OSWstate is expressed, for example, by 0 when OSW 21 is switched to #0system, and by 1 when OSW 21 is switched to #1 system, as in the case oftable contents in FIG. 10B.

On receipt of OSC transmitted from the upstream station, OSCprocessor/controller 57 in the downstream station N1 supplies thepreceding notification of switchover included in OSC to OSCprocessor/controller 58 of the station concerned. Meanwhile, as to theSW state included in this OSC, after adding the state of OSW 32 (OSW onthe receiving side, for example, 0 or 1) to this SW state, OSCprocessor/controller 57 supplies this added SW state to OSCprocessor/controller 58 and LAMP 55 in the station concerned.

As a result of the state (for example, 0 or 1) of OSW 32 (OSW on thereceiving side) of the station concerned being added to the SW state,the new SW state becomes {the state of OSW 21 on the upstream station,the state of OSW 32 on the downstream station}. Also, as a result of theSW state being supplied to LAMP 55, LAMP 55 can select the ASEcorrection table corresponding to the supplied SW state, and the ASEcorrection factor in the selected ASE correction table.

OSC processor/controller 58 rewrites the preceding notification ofswitchover supplied from OSC processor/controller 57 into OSC. Also, OSCprocessor/controller 58 adds the state (for example, 0 or 1) of OSW 41(OSW on the transmitting side) of the station concerned to the SW statesupplied from OSC processor/controller 57, and writes the added SW stateinto the SW state in OSC. OSC processor/controller 58 then sends thisOSC signal to the neighboring downstream station N2 (step S32).

The same process as OSC processor/controllers 57, 58 in the downstreamstation N1 is also executed in the downstream stations N2-Nn−1 (stepS33). In the OSC transmitted to downstream station Nn (receivingterminal station 1 ₂), the preceding notification of switchover and theSW state covering the switches from OSW 21 in the upstream station toOSW 41 in the downstream station Nn−1 are included.

OSC processor/controller 19 provided in the downstream station Nn addsthe state of OSW 22 of the station concerned to the received SW stateincluded in OSC, and supplies the added SW state to RAMP 14. With this,RAMP 14 can select the ASE correction table corresponding to the givenSW state, and the ASE correction factor in the selected ASE correctiontable.

I After transmitting the preceding notification of switchover and the SWstate, the upstream station sends a switchover trigger signal to thedownstream stations N1 (step S2), and each the downstream stationN1-Nn−1 successively transfer the switchover trigger signal to theneighboring downstream station (steps S34, S33). Finally, the downstreamstation Nn receives the switchover trigger signal.

In synchronization with transmission/reception of this trigger signal,the upstream station and the downstream station N1 switches OSW (stepsS3, S4), and also, each optical amplifier in the downstream stationsN1-Nn selects the corresponding ASE correction table and the ASEcorrection factor, and performs the ASE correction after the switchoverusing the selected ASE correction factor (steps S36-S38).

Here, while this switchover is in execution, the downstream station N1performs the dummy light transmission processing described earlier.

When the switchover is performed in the other transmission lines, theprocess similar to the above is performed.

According to this third method, the ASE correction factor is determinedin advance, and by selecting this ASE correction factor, the ASEcorrection after the switchover can be performed. Accordingly, thecalculation time to obtain ASE correction amount, etc. is not necessaryat the time of switchover, and thus the processing time is reduced.

Additionally, in the ASE correction table, the ASE correction amount maybe specified in place of the ASE correction factor. Or, the target levelmay be specified in this ASE correction table.

<Pre-Emphasis Control>

As described earlier, each attenuation factor (or attenuation amountwhen expressed by dB) of VAT 101-10 m is adjusted by optical spectrumanalyzer 15 in the sending terminal station 1, so as to control thesignal quality (OSNR) of each wavelength (each channel) received in thereceiving terminal station 1 to be uniform. Thus, pre-emphasis controlis performed.

However, when the transmission line is switched and a wavelengthdependent loss (WDL) of the transmission line and gain uniformity of therelated optical amplifier are varied, the way of the tilt generation inthe WDM optical signal may possibly be varied. This producesnon-uniformity in the signal quality (OSNR) of each wavelength, and anerror may occasionally be produced in a particular wavelength signal.

As a method for correcting this non-uniformity to be uniform, one methodis to adjust a pre-emphasis amount. Namely, when the switchover isperformed, the sending terminal station 1 performs the pre-emphasiscontrol to readjust the attenuation factor (weight) of the signal levelof each wavelength (each channel), so that the OSNR of each wavelengthsignal in the receiving terminal station 1 becomes uniform.

As to this pre-emphasis control, there are three methods describedbelow.

(1) First Method of Pre-Emphasis Control

The first method of the pre-emphasis control is that peak power (peaklevel) of each reception channel signal after the switchover is measuredin the receiving terminal station. Based on the measured peak level, arequired increase or decrease of the attenuation factors of VAT in thesending terminal station is notified to the sending terminal station.The sending terminal station then readjusts the attenuation factor ofVAT based on this notification.

FIG. 14 shows a sequence diagram illustrating a processing flow of thefirst method of the pre-emphasis control. This sequence diagramexemplarily shows the processing performed when the transmission line inuse between terminal station 1 ₁ (upstream station) and relay station 2₁ (downstream station N1) is switched from #0 system to #1 system. Thesame numerals are assigned for the same process as that shown in FIGS.5, 7, etc., and the detailed description is omitted.

At the time of initiating this WDM optical communication system, opticalspectrum analyzer 16 provided in the receiving terminal station 1 ₂(downstream station Nn) measures both OSNR and a peak level (peak power)of each channel in the reception signal (wavelengths λ1-λm), andsupplies the OSNR and the peak level of each channel to OSCprocessor/controller 19. OSC processor/controller 19 stores the OSNR andthe peak level supplied. Thereafter, when the number of channels (m) isvaried, the OSNR and the peak level of each channel are also measured byoptical spectrum analyzer 16, and stored in OSC processor/controller 19.

After that, when the transmission line between the upstream station andthe downstream station N1 is to be switched from #0 system to #1 system,a preceding notification of switchover and a switchover trigger signalare successively transmitted from the upstream station to the downstreamstations N1-Nn (step S1, S2). Thereafter, the upstream station and thedownstream station N1 switch OSW. Here, at the time of the switchover,the aforementioned dummy light transmission process is performed, andafter the switchover, the aforementioned ASE correction is executed.

After the switchover, optical spectrum analyzer 16 measures the peaklevel of each channel (each wavelength), and supplies this peak level toOSC processor/controller 19. In case that the notification of the ASEcorrection completion is sent, preferably, this peak level measurementis performed after the downstream station Nn receives the notificationof the ASE correction completion.

OSC processor/controller 19 obtains a difference value between the peaklevel measured after switchover and the peak level stored at the time ofthe system initiation or the modification of the number of channels, ona channel-by-channel basis. OSC processor/controller 19 then writes thisdifference value into a pre-emphasis information field, and transmits tothe upstream station (step S41).

OSC processor/controller 19 in the upstream station executes thepre-emphasis control based on the OSC signal transmitted from thedownstream station Nn, so as to control optical spectrum analyzer 15 tomodify each attenuation factor of VAT 10 ₁-10 _(m) (step S42) Namely,OSC processor/controller 19 increases the attenuation factor of VAT 10_(i), so that the output level of channel i is decreased by di, when thepre-emphasis information corresponding to channel i (i=1−m) has adifference value di (>0). On the other hand, when the difference valueis −di (<0), OSC processor/controller 19 decreases the attenuationfactor of VAT 10 _(i), so that the output level of channel i isincreased by di. Meanwhile, when the difference value equals to 0, theattenuation factor is not varied.

Such pre-emphasis control is based on the assumption that, the ASEcomponent is constant, and as to a channel of which peak level (=signalcomponent+ASE component) becomes greater than the peak level obtained atsystem initialization, etc., it is regarded that the OSNR becomesbetter, and, as to a channel of which peak level (=signal component+ASEcomponent) becomes less than the peak level obtained at systeminitialization, etc., it is regarded that the OSNR becomes deteriorated.In such a way, by obtaining a difference value of the peak levels andperforming the pre-emphasis control, signal quality can be restoredwithin a shorter time, as compared to a pre-emphasis control method ofmaking OSNR uniform after obtaining OSNR.

Execution of such feedback control produces the OSNR of each channelafter the switchover to be substantially same as the OSNR having beenadjusted at the time of the system initiation or the modification of thenumber of channels. As a result, it becomes possible to preventdeterioration of the optical signal reception quality resulting from theswitchover of the transmission lines, and to prevent generation of asignal error.

Additionally, the similar processing is performed when such a switchoveris performed on any other transmission lines.

(2) Second Method of Pre-Emphasis Control

The second method of the pre-emphasis control is that the sendingterminal station 1 retains VAT attenuation factors corresponding to theOSW states of the stations from the sending terminal station 1 to thereceiving terminal station 1, in the form of table (attenuation factortable), and after the switchover, the VAT attenuation factors arereadjusted based on this table.

FIG. 16 shows an example of the attenuation factor table. Thisattenuation factor table specifies each attenuation factor of VAT 10₁-VAT 10 _(m) corresponding to each OSW state of the stations from thesending terminal station (in this case, terminal station 1 ₁) to thereceiving terminal station (in this case, terminal station 1 ₂), as issimilar to the ASE correction table shown in FIG. 13A. This attenuationfactor table is retained in either optical spectrum analyzer 15 or OSCprocessor/controller 19. Each attenuation factor is obtained throughexperiments, system test operation, actual system operation, etc.

FIG. 15 shows a sequence diagram illustrating a processing flow of thesecond method of the pre-emphasis control. By way of example, FIG. 15shows processing in case that the transmission line between thedownstream station N1 (relay station 2 ₁) and the downstream station N2(relay station 2 ₂) is switched over.

According to this second method, SW states are transmitted in a similarway to the aforementioned third method of the ASE correction. The SWstates are transmitted to both directions, upstream from the downstreamstation N1 in which the transmission line is switched (i.e. to theupstream station), and downstream (i.e. to the downstream stationsN2-Nn)(step S51).

The station having received the SW states transmitted upstream adds theown OSW state to the received SW states, and then transmits the added SWstates further upstream. Finally, the SW states are transmitted to thesending terminal station (terminal station 1 ₁ in FIG. 15) (step S51).Meanwhile, the station having received the SW states transmitteddownstream also adds the own SW states to the received SW states, andthen transmits the added SW states further downstream. Finally, the SWstates are transmitted to the receiving terminal station (terminalstation 1 ₂ in FIG. 15) (step S51). The receiving terminal station addsthe own OSW states to the SW states, and transmits the added SW statesto the sending terminal station (step S55).

After the SW states (and the preceding notification of switchover) aretransmitted, sending and receiving the switchover trigger signal (stepS52) trigger the switchover (steps S53, S54).

The sending terminal station acquires the OSW states of the entiresystem by receiving and combining the SW states having been transmittedboth upstream and downstream. Thus, the sending terminal stationdetermines the attenuation factors of VAT 10 ₁-10 _(m) corresponding tothe OSW states of the entire system, and performs the pre-emphasiscontrol (step S56). For example, in FIG. 16, when the SW states are{001100}, attenuation factors α13-αm3 are selected, and theseattenuation factors are set into VAT 10 ₁-10 _(m) by optical spectrumanalyzer 15, respectively.

Here, in case that the notification of the ASE correction completion istransmitted to the sending terminal station, it may be possible for thesending terminal station to perform the pre-emphasis control on receiptof this ASE correction completion notification. Further, the similarprocessing is performed when such a switchover is performed on any othertransmission lines.

According to this second method, since the attenuation factors areobtained using the attenuation factor table, the pre-emphasis controlcan be performed at high speed after the switchover. Further, thissecond method is effective when the OSNR of the reception channel signalcannot be measured constantly in the receiving terminal station.

(3) Third Method of Pre-Emphasis Control

The third method of the pre-emphasis control is that, the peak level ofeach channel, which is measured at system initialization or themodification of the number of channels in the aforementioned firstmethod, is measured again immediately before the switchover.

Namely, in FIG. 14, on receipt of the preceding notification ofswitchover, downstream station Nn measures the peak level of eachchannel, and supplies this peak level to OSC processor/controller 19.OSC processor/controller 19 stores the supplied peak level.

Thereafter, when the switchover is performed, optical spectrum analyzer16 measures the peak level of each channel (each wavelength) after theswitchover, and supplies the measured peak level to OSCprocessor/controller 19. In case that the notification of the ASEcorrection completion is transmitted, preferably, this measurement isperformed after the downstream station Nn receives the notification ofthe ASE correction completion.

Thereafter, the steps S41 and S42 in the first method described earlierare performed.

According to this third method, it becomes also possible to avoiddeterioration of the reception quality of the optical signal caused bythe transmission line switchover.

Additionally, it may also be possible for optical spectrum analyzer 16to measure the peak level of each channel at certain intervals (forexample, 100 msec, 200 msec, or the like). With this, even when theswitchover is performed without transmission of the precedingnotification of switchover because of a sudden transmission linefailure, the peak level of each channel before the switchover can bemeasured. Further, the similar processing is performed when such aswitchover is performed on any other transmission lines.

<Connection By Use of Optical Fibers of Different Kinds>

A dispersion amount of an optical fiber increases in the order of DCF(dispersion compensation fiber), NZDSF (non-zero dispersion shiftfiber), and SMF (single mode fiber). Therefore, in case NZDSF is used astransmission line, it is necessary to keep the optical signal levellower, being input to the NZDSF from either the sending terminal station1 or relay station 2, than that being input to the SMF. Accordingly, incase that NZDSF is employed for one of the main transmission line andthe backup transmission line, while SMF is employed for the othertransmission line, and if both transmission lines produce substantiallythe same loss, then the levels to be output to the transmission linesare confined to the output level to the NZDSF transmission line, andwhen the transmission line is switched to the SMF transmission line, theoutput level thereto is maintained to the same level as before.

However, when the transmission lines are duplicated, one of thetransmission lines (for example, backup transmission line) is, in manycases, an alternative route having a longer distance than the other.Under such a condition, when SMF is employed for the alternative routeand NZDSF is employed for the other route, although the loss on the SMFside is larger, optical signals can be transmitted by increasing theoutput level of the SMF transmission line.

As such, adjustment of the output level on the transmitting incompliance with the losses of #0 system and #1 system enables not onlyASE correction but also the switchover between the fibers of differentkinds. Further, because of dispersion amount of SMF different from thatof NZDSF, such a configuration as shown in FIG. 17 is also possible.

More specifically, in case that SMF is employed for #0 system, and thatNZDCF is employed for #1 system, a DCF 92 is disposed on the #0 systemto compensate the difference in the dispersion amount between bothfibers. Further, a DCF 95 is disposed on the output side of opticalamplifier (AMP) 94 to compensate the dispersion amount of NZDSF.

The embodiment having been described above is illustrated in regard tothe WDM optical communication system. However, the present invention (inparticular, the invention with regard to the dummy light transmissionprocessing and the ASE correction) is applicable to a single-wavelengthoptical communication system.

The above description is based on an optical communication system havingrelay station 2. However, the present invention is also applicable to asystem having only a sending terminal station and a receiving terminalstation, without any relay station 2, needless to say.

Further, in regard to the data included in the OSC signal (refer to FIG.4), instead of multiplexing the data into a WDM optical signal fortransmitting to relay station 2, terminal station 1, etc., it may alsobe possible to transmit to the relay station and the receiving stationthrough a different line. For example, by providing central supervisoryequipment separately from terminal station 1 and relay station 2, thedata included in the OSC signal is transmitted to this centralsupervisory equipment, and then this data is transmitted from thecentral supervisory equipment to terminal station 1 and relay station 2.

INDUSTRIAL APPLICABILITY

The present invention is applicable to an optical communication systemhaving plural sets of optical transmission equipment in whichneighboring two optical transmission equipment sets are connected by themain transmission line and the backup transmission line, and opticalsignals is transmitted through one of the transmission lines, andoptical transmission equipment (sending equipment (sending station)),relay equipment (relay station), receiving equipment (receiving station)in the optical communication system. In particular, the presentinvention is used for a WDM optical communication system and opticaltransmission equipment in the WDM optical communication system.

The foregoing description of the embodiments is not intended to limitthe invention to the particular details of the examples illustrated. Anysuitable modification and equivalents may be resorted to the scope ofthe invention. All features and advantages of the invention which fallwithin the scope of the invention are covered by the appended claims.

1. An optical communication system having a sending unit transmittingmultiplexed optical signals, in which optical signals are multiplexedwith wavelength division multiplexing, to a receiving unit througheither one of a main transmission line or a backup transmission line,and a receiving unit receiving the optical signal transmitted from asending unit from either one of the main transmission line or the backuptransmission line, the said sending unit comprising: a pre-emphasissection adjusting signal levels of each wavelength; a transmittermultiplexing each wavelength signal of which level is adjusted by thepre-emphasis section, and sending the multiplexed wavelength signal; anda controller controlling the pre-emphasis section to adjust eachwavelength signal level, based on control data transmitted from thereceiving unit for adjusting each wavelength signal level, and the saidreceiving unit comprising: a measurement section measuring signalquality of each wavelength included in received optical signals; amemory storing the initial value of the signal quality of eachwavelength measured by the measurement section before the transmissionline is switched over either from the main transmission line to thebackup transmission line or from the backup transmission line to themain transmission line; and a transmitter ordering the measurementsection to measure the signal quality of each wavelength included inoptical signals received after a switchover when a transmission lineswitchover is performed, and generating control data based on both theinitial quality value and the post-switchover quality value, and sendingthe generated control data to the sending unit.
 2. The opticalcommunication system according to claim 1, wherein the signal qualitymeasured by the measurement section is a peak level of each wavelengthsignal, and control data are difference values between the initial valueand the post-switchover value, and a controller in the sending unitdecreases the wavelength signal level by the difference value ofcorresponding wavelength when the difference value is positive, andincreases the wavelength signal level by the difference value ofcorresponding wavelength when the difference value is negative.
 3. Theoptical communication system according to claim 1, wherein the initialvalue is measured when operation of the receiving unit is initiated orwhen the number of the wavelengths is changed.
 4. The opticalcommunication system according to claim 1, wherein the initial value ismeasured immediately before a switchover.
 5. The optical communicationsystem according to claim 4, wherein the initial value is obtainedimmediately before the switchover among the values measured at certainintervals before the switchover.
 6. The optical communication systemaccording to claim 1, further comprising: one or more relay unitsprovided between the sending unit and the receiving unit, wherein, thesending unit and the neighboring relay unit, and neighboring relay unitswhen two or more relay units exist, are interconnected by the maintransmission line and the backup transmission line, and the opticalsignal is transmitted from the sending unit to the receiving unitthrough the relay units on either one of the main transmission line andthe backup transmission line provided between each station, and atransmitter in the receiving unit performs the said process when atleast one transmission line between stations is switched from the maintransmission line to the backup transmission line, or from the backuptransmission line to the main transmission line.
 7. A sending unitsending optical signals, in which optical signals are multiplexed withwavelength division multiplexing, to a receiving unit through either oneof a main transmission line or a backup transmission line, said sendingunit comprising: a pre-emphasis section adjusting each wavelength signallevel; a transmitter multiplexing and transmitting each wavelengthsignal of which level is adjusted by the pre-emphasis section; and acontroller controlling the pre-emphasis section to adjust eachwavelength signal level, based on control data sent from the receivingunit for adjusting each wavelength signal level.
 8. A receiving unitreceiving a multiplexed optical signal from either one of a maintransmission line or a backup transmission line, in which opticalsignals are multiplexed with wavelength division multiplexing and sentfrom a sending unit through a main transmission line or a backuptransmission line, the said receiving unit comprising: a measurementsection measuring signal quality of each wavelength included in thereceived optical signal; a memory storing an initial value of the signalquality of each wavelength measured by the measurement section beforethe transmission line is switched over either from the main transmissionline to the backup transmission line or from the backup transmissionline to the main transmission line; and a transmitter, on occurrence ofa switchover, ordering the measurement section to measure signal qualityof each wavelength included in the optical signal received after theswitchover, and transmitting to the sending unit control data by whichthe sending unit controls a transmission level of each wavelength, basedon both the initial quality value and the post-switchover quality value.9. A sending unit transmitting an optical signal, in which opticalsignals are multiplexed with wavelength division multiplexing, to areceiving unit through either one of a main transmission line or abackup transmission line, the said sending unit comprising: apre-emphasis section adjusting each wavelength signal level; atransmitter multiplexing and transmitting each wavelength signal ofwhich level is adjusted by the pre-emphasis section; a memory storingcontrol data which include adjustment amounts of the pre-emphasissection correspondingly to combinations of the transmission line statesindicating which of the main transmission line and the backuptransmission line is in use for the optical signal transmission withrespect to the transmission lines located between the sending unit andthe receiving unit; and a controller controlling the pre-emphasissection to adjust each wavelength signal level based on the transmissionline states and the control data.
 9. A sending unit transmitting anoptical signal, in which optical signals are multiplexed with wavelengthdivision multiplexing, to a receiving unit through either one of a maintransmission line or a backup transmission line, the said sending unitcomprising: a pre-emphasis section adjusting each wavelength signallevel; a transmitter multiplexing and transmitting each wavelengthsignal of which level is adjusted by the pre-emphasis section; a memorystoring control data which include adjustment amounts of thepre-emphasis section correspondingly to combinations of the transmissionline states indicating which of the main transmission line and thebackup transmission line is in use for the optical signal transmissionwith respect to the transmission lines located between the sending unitand the receiving unit; and a controller controlling the pre-emphasissection to adjust each wavelength signal level based on the transmissionline states and the control data.